Small microwave elements have been designed in the past by utilizing a high-Q microwave cavity with an air dielectric by fabricating the cavity as a metallic housing with a microwave source mounted within that cavity. The cavity further normally has an opening that is called an iris. The cavity normally further mounts a metallic, horn-shaped element that acts as the antenna for the device. This type of microwave element can be used in a multitude of applications, such as intrusion detection, proximity sensing, and similar functions. Microwave energy is propagated from the microwave element and the microwave element further uses a Doppler effect and the self-mixing characteristics of the microwave source, or uses the injection of a modulating microwave energy into the microwave cavity from a source separate from the microwave generator for these various applications.
The use of a microwave device of the type just described is limited because of the cost of fabrication of the cavity, iris, and antenna along with the physical size and power requirements of the device itself. In the general class of devices just outlined, the microwave cavity normally supports a Gunn diode, Impatt diode or a Baritt diode, which diodes are known in the art. These diodes are microwave energy sources when properly energized from a direct current source of potential. These devices operate somewhere in the region of 8 to 17 or more gigahertz. In the prior art devices, a Gunn diode, Impatt diode or a Baritt diode is mounted at a one-half wave length location within a high-Q microwave cavity in such a manner that the mounting acts as a series inductance to the high frequency, as well as, simultaneously acting as a capacitance coupling to the walls of the microwave cavity. With this arrangement it is possible to energize the Gunn diode, Impatt diode or the Baritt diode with a relatively low voltage direct current, and cause the diode to oscillate at a microwave frequency. The series inductance blocks the microwave energy from leaving the cavity, while the capacitive element couples the energy to the cavity. The energy is then propagated through the iris to the air horn or antenna where it is radiated into the atmosphere. The design of the device and its air horn determine the antenna pattern. This type of device can either be Doppler operated to measure distance or proximity using the self-mixing characteristics of the energy source, or can be modulated by the injection of additional energy at an appropriate point in the microwave cavity.
This type of device has the disadvantages of size, cost, and the generation of microphonic type noise. The microphonic type noise is a function, to some extent, of the mechanical stress or relative movement of the cavity, iris and air horn. If this structure could be made more rigid, the noise in the system could be reduced.